1. Field of the Invention
This invention relates to a short-circuit protective circuit for uninterruptible power supply equipment, and more particularly which comprises not only an inverter circuit but also a bypass circuit connected with a commercial power supply and in parallel to the inverter circuit.
2. Description of the Prior Art
A conventional short-circuit protective circuit for uninterruptible power supply is illustrated in the form of schematical block diagram in FIG. 1, in which reference numerals designate 1, an inverter circuit; 2, a thyristor switch connected, through a line 3, between the inverter circuit 1 and a plurality of feeders; 4, fuses attached to feeders; 5, a current sensor disposed approximately at the middle position of the line 3; 6, a commercial power supply to a bypass circuit 11 connected to the line 3 through a bypass thyristor switch 7; 8, an overcurrent detector connected to the current sensor 5; 9, an AND circuit which outputs a logical product of a signal S.sub.1 generated when there is a commercial supply voltage synchronous with the output voltage of the inverter circuit 1 and the output signal of the overcurrent detector 8; 10, a flip-flop for discriminating whether power is supplied through the bypass circuit 11 or through the inverter circuit 1; 12 and 13, gate drive circuits; 14, a turn-off delay circuit for delaying turn-off operation of the bypass thyristor switch 7; 15, an interlock circuit for bringing interlock with signal S.sub.2 discerning that the inverter circuit 1 is normal to allow change of power supply from that through the bypass circuit 11 to that through inverter circuit 1; and S.sub.3, a command signal which progressively advances the phase of the inverter to shift a load when a power supply source supplied to the load is switched from the bypass circuit 11 to the inverter circuit 1.
The operation of the prior art circuit will be described below:
Now, when load is supplied with a power through the inverter circuit 1, a short-circuit accident at the load part (if it happens) results in appearance of overcurrent at the output of the inverter circuit 1. The overcurrent is detected by the overcurrent detector 8 associated with the current sensor 5. The detection signal together with the aforesaid signal S.sub.1 is input into the AND circuit 9. When both signals appear on the inputs, the flip-flop 10 is triggered, and causes the thyristor 2 connected to the inverter circuit 1 to turn OFF through the intermediation of the gate drive circuit 13, and at the same time, to turn ON the bypass thyristor switch through the gate drive circuit 12 and the turn-off delay circuit 14. Consequently the connection of the load with power supply through the inverter 1 is changed to that through the bypass circuit 11. When the bypass thyristor switch 7 turns on, therefore, a large short-circuit current from the commercial power supply 6 flows through a fuse 4 of the short-circuit feeder and blows it. Thus, once the output current returns to the normal value, or the overcurrent detector 8 turns OFF, the flip-flop 10 is reset in response to the invert signal, S.sub.2, and the output of the AND circuit 9. As the result, the thyristor switch 2 turns ON, and the bypass thyristor switch 7 turns OFF. In this case, owing to the large impedance of the power supply through the inverter circuit 1, upon changing power supply to the load from through the bypass circuit 11 to through the inverter circuit 1, the inverter acts to delay timing to turn OFF the bypass thyristor switch 7, while advancing gradually the phase in accordance with the above-mentioned signal S.sub.3.
In the conventional short-circuit protective circuit, as described above, it is necessary for the load to reconnect with the inverter circuit 1 after the fuse 4 has blown, and in turn complicated sequential control of inverter is required. This is a problem encountered in the prior art. Another problem arises in the respect that change to the connection of the load with the bypass circuit 11 may cause a fault of the bypass thyristor switch 7 due to short-circuit current, resulting in the bypass thyristor switch 7 remaining in a conducting state. In this case, reswitching to the connection with the inverter circuit 1 is impossible because of being impossible to turn off the bypass thyristor switch 7. The third problem is that, after turn-on of the thyristor switch 2, gradual increase in transverse current and system down may occur.